[1439-36-7]  · C21H19OP  · Acetylmethylenetriphenylphosphorane  · (MW 318.36)

(stabilized ylide for Wittig alkenation of aldehydes; precursor to vinylphosphonium salts; synthesis of 1,2,3-triazoles and pyrazoles; cyclocondensation on a,b,g,d-unsaturated carbonyls)

Physical Data: mp 203-205 °C.

Solubility: sol methanol, chlorinated solvents, low pH water.

Form Supplied in: fine white powder.

Preparative Methods: treatment of Triphenylphosphine with chloro- or Bromoacetone provides the phosphonium salt (eq 1). Deprotonation with a weak base such as bicarbonate in cold water provides the crude ylide which may by isolated by filtration.

Handling, Storage, and Precautions: may be handled in atmosphere for short periods; will decompose in water at pH > 9.

Wittig Alkenations.

The title ylide is primarily used in the literature to synthesize a,b-unsaturated methyl ketones in a Wittig reaction (eq 2).

This reagent is a close relative to the related stabilized ylide (Methoxycarbonylmethylene)triphenylphosphorane which will provide an a,b-enoate, while the title reagent provides the corresponding enone from the same aldehyde.1 The reaction is generally trans selective in nonpolar solvents like methylene chloride, chloroform, and aromatics. It is rather quick but not as selective in alcoholic solvents. The stabilized ylides are also rather unreactive to most ketones. The Horner-Emmons-Wadsworth alkenation utilizing a phosphonate ester is used for the same purpose and will react with ketones to provide the trisubstituted alkenes in high yield.

The title reagent is also a critical substrate in the synthesis of a variety of substituted phosphorus compounds. Cooke2 has carried out metalations at the methyl group with n-Butyllithium followed by treatment with an alkyl halide to provide the substituted ylide (eq 3). He follows with a hydrolysis of the phosphonium portion to provide a good yield of the saturated ketone. Other workers3 have also used this synthesis as a route to substituted enones.

Vinylphosphonium Salts.

The neutral form of the ylide is also mildly nucleophilic. It is, however, an ambident nucleophile. It will react with alkyl halides although only methyl or ethyl iodide or benzyl bromide provide synthetically useful yields (eq 4).4 The primary product is that of O-alkylation but this product rearranges on heating to the C-alkyl product.5 It has also been shown that the O-alkyl products, which are vinylphosphonium salts, will also react with sodium alkoxide to form alkoxyvinyldiphenylphosphine oxides.6

Heterocycle Synthesis.

The nucleophilicity of phosphorus ylides is also apparent in their reactions with 1,3-dipoles or their equivalents to generate heterocycles. Reaction with alkyl or aryl azides provides a high yield route to 1,2,3-triazoles (eq 5).7 The initial step of the postulated mechanism is similar to the first step of the Wittig reaction, namely attack of the nucleophilic carbon of the ylide onto the electrophilic end nitrogen of the azide to form a betaine intermediate. Attack of the internal nitrogen onto the carbonyl to form a five-membered ring, followed by elimination of triphenylphosphine oxide, provides the triazole. In similar fashion, treatment of the ylide with hydrazonyl halides provides a useful route to a variety of substituted pyrazoles (eq 6).8 This mechanism is similar to the triazole formation in that the nucleophilic ylide attacks the electrophilic carbon of the hydrazonyl halide, displacing chlorine. Proton abstraction by added Triethylamine from the hydrazone, followed by cyclization and elimination of triphenylphosphine oxide, provide the pyrazole.

One last application is in the reaction of the ylide at the d-position of an a,b,g,d-unsaturated carbonyl system (eq 7). This reaction begins with addition of the ylide to form an enolate/betaine which then attacks the carbonyl of the ylide to form a six-membered ring followed by elimination of triphenylphosphine oxide.9 Although the reaction goes in low yield it provides an interesting synthesis of some bicyclic systems.

1. Harris, T. M.; Harris, C. M. OR 1969, 17, 155.
2. Cooke, M. P. Jr. JOC 1973, 22, 4082.
3. Chamberlin, K. S. and LeGoff, E. SC 1978, 8, 579.
4. Nesmeyanov, N. A.; Berman, S. T.; Petrovskii, P. V.; Lutsenko, A. I.; Reutov, O. A. ZOR 1977, 13, 2465.
5. Nesmeyanov, N. A.; Berman, S. T.; Reutov, O. A. ZOR 1975, 11, 2845.
6. Ruder, S. M.; Norwood, B. K. TL 1992, 33, 861.
7. (a) Harvey, G. R. JOC 1966, 31, 1587. (b) Ykman, P.; Mathys, G.; L'Abbé, G.; Smets, G. JOC 1972, 37, 3213.
8. Dalla Croce, P. AC(R) 1973, 63, 867.
9. Flitsch, W.; Gesing, E. R. F. CB 1981, 114, 3146.

Onorato Campopiano

DuPont Agricultural Products, Wilmington, DE, USA

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